COMPOSITIONS FOR JOINING AND ASSEMBLING PARTS MADE OF SiC-BASED MATERIALS

ABSTRACT

A method for joining, assembling, at least two parts made of silicon carbide-based materials by non-reactive brazing is provided. According to the method, the parts are contacted with a non-reactive brazing composition, the assembly formed by the parts and the brazing composition is heated to a brazing temperature sufficient to melt the brazing composition totally or at least partly, and the parts and brazing composition are cooled to that, after solidification of the brazing composition, a moderately refractory joint is formed; wherein the non-reactive brazing composition is an alloy comprising, in atomic percentages, 45% to 65% silicon, 28% to 45% nickel and 5% to 15% aluminium. A brazing composition as defined above is provided. A brazing paste, suspension comprising a powder of said brazing composition and an organic binder as well as a joint and assembly obtained the foregoing method are also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 13/635,353, filed Sep. 20, 2012 which is the U.S. National Phase ofInternational Application No. PCT/EP2011/053690, filed Mar. 11, 2011,designating the U.S. and published as WO 2011/113758 on Sep. 22, 2011which claims the benefit of French Patent Application No. 10 51870 filedMar. 16, 2010.

TECHNICAL FIELD

The present invention relates to a method for joining, assembling, partsmade of silicon carbide-based materials by non-reactive brazing with anon-reactive brazing composition, for the purpose in particular ofpreparing components entirely based on silicon carbide.

The joining, assembling, method according to the invention is generallyimplemented at a temperature not exceeding 1150° C., preferably between1040° C. and 1150° C.

The invention further relates to brazing compositions and to a joint andassembly obtained by this method, an assembly whose maximum temperatureof use of the assembled component is generally between 950° C. and 980°C.

By <<silicon carbide-based>> materials is generally meant a materialwhose SiC content is equal to or higher than 50% by weight, preferablyequal to or higher than 80% by weight, further preferably it is 100% byweight, in this latter case it can be said that the material consists oris composed of silicon carbide.

The silicon carbide may be in the form of fibres of silicon carbide orof a powder of silicon carbide sintered or bonded via a ceramic binder.

These silicon carbide-based materials may notably be pure siliconcarbide such as pure α (α-SiC) or pure β (β-SiC) silicon carbide,substrates made of silicon carbide infiltrated with silicon (SiSiC), orSiC-based composite materials such as composite materials with siliconcarbide fibres and/or matrix.

The technical field of the invention can be defined as the brazing at animplementation temperature (temperature equivalent to the brazingplateau, hold, temperature) generally not exceeding 1150° C., preferablyat a temperature of 1040° C. to 1150° C.

The assemblies concerned by the present invention are thereforegenerally considered to be <<moderately refractory>> i.e. the maximumtemperature of use of these assemblies is generally of the order of 950°C. to 980° C.

These assemblies can enter into the production of components of complexshapes requiring good mechanical strength between the siliconcarbide-based substrates and optionally a satisfactory seal either sideof the joint.

BACKGROUND

It is known that it is difficult to fabricate parts of large size madeof ceramic, in particular made of SiC. The tolerances after sinteringthe primary components made of silicon carbide of large size are illcontrolled and the machining of these components is unacceptable forcost-related reasons.

In addition, and for the same reasons, it is generally difficult tofabricate parts of complex shape with silicon-based compounds such assilicon carbide.

It is therefore often preferable to fabricate parts or structures oflarge size and/or of complex shape from elements in ceramic of simpleshape and/or of small size, and then to assemble these elements to formthe final structure.

Said technique is particularly necessary for fabricating structures ofheat exchanger type and structural components in silicon carbide havinga temperature of use possibly reaching up to for example 900° C., even1000° C.

On account of the high temperatures, close to 900° C. to 1000° C. forexample, used in applications of ceramics such as silicon carbide, thejoining of these ceramics by bonding with organic adhesives is excludedsince the temperatures of use of this type of assembly cannot exceed200° C. at the most.

Purely mechanical assemblies, for example by stapling or screwing, onlyensure partial, random contact between the parts. The assemblies thusobtained cannot be leak tight. The mechanical strength is only ensuredby the staples and screws, which is limited. To ensure good mechanicalstrength of the joint, it is essential to create good adhesion betweenthe parts to be joined, which is not possible with screws or staples.

Additionally, conventional joining techniques by welding having recourseto an energy beam with or without a supply of metal (TIG, electron orlaser welding) and involving the partial melting of the parts to bejoined cannot be used for assembling ceramics since it is not possibleto melt a substrate or a part in ceramic, and in particular sincesilicon carbide decomposes before melting.

Usual techniques for obtaining refractory assemblies of ceramics aresolid phase diffusion bonding and joining by sintering or co-sintering.

For assembly by diffusion bonding, a pressure is applied at hightemperature between the interfaces to allow atomic inter-diffusionbetween the two substrates. The temperature must always remain lowerthan the melting point of the least refractory material, and there istherefore no liquid phase in the system. This type of joining isobtained either under a press in single direction, or in an isostaticchamber. Diffusion bonding is well adapted for the joining of two metalalloys and very little adapted for the joining of ceramic materials,since the atoms forming the ceramic scarcely diffuse at the joint. Inaddition, the method is prohibitive from a mechanical viewpoint since itrequires placing under compression porous, fragile substrates andmaterials such as silicon carbide composites which risk being highlydamaged under this mechanical compressive loading.

The joining by sintering or co-sintering of parts made of SiC alsorequires high pressures but in addition high temperatures and long holdtimes since this process is based on the principle of inter-diffusionbetween the SiC elements.

In other words, solid phase diffusion bonding and joining by sinteringhave the disadvantage of being restrictive from an implementationstandpoint since:

-   -   for solid phase diffusion bonding, the shape of the parts must        remain simple if uniaxial pressing is used, or else it requires        complex tooling and preparation for example entailing the        fabrication of a jacket, vacuum sealing, hot isostatic pressing,        final machining of the jacket if HIP is used (Hot Isostatic        Pressing).    -   for co-sintering or joining by sintering the problems remain the        same (shape of the parts, complex implementation) with, in        addition, the need to control the sintering of a filler powder        to be inserted between the two materials to be joined.    -   these two techniques additionally require the use of long hold        times (one to several hours) at high temperature since the        processes used have recourse to solid state diffusion.

It follows from the above, and to summarize, that in order to guaranteegood mechanical strength in particular and optionally satisfactorysealing of the assembly, only those processes using a liquid phase suchas brazing can be envisaged.

Brazing is a low-cost technique, easy to perform and is the mostcommonly used. Parts of complex shape can be prepared using brazing, andbrazing operations are limited to placing between the parts to bejoined, or in the vicinity of the joint between the two parts, a filleralloy called a braze alloy and melting this alloy which is capable ofwetting and spreading over the interfaces to be joined, filling thejoint between the parts. After cooling the brazing alloy solidifiesensuring the cohesion of the assembly.

Most brazing compositions for parts in silicon carbide-based materialsare insufficiently refractory. These are generally brazing compositionsformed by metal alloys having a melting point that is lower even muchlower than 1000° C. Said melting temperature is distinctly insufficientfor applications at temperatures in the region of 900° C. or 1000° C.,for example from 950° C. to 980° C.

Also, most chemical elements which form part of these metal brazingcompositions are highly reactive with silicon carbide on and after 500°C. and lead to fragile compounds.

As a result, for brazing at higher temperatures generally above 1000°C., said brazing compositions or braze alloys would chemically attackthe silicon carbide-based materials not only during the brazingoperation but also during functional use by solid state diffusion.

It is also pointed out that the least reactive alloys are also the leastrefractory, such as the AgCuTi alloy for example with Ag—Cu matrix andactive Ti element in low concentration. For the applications moreparticularly concerned by the invention, which are those of moderatelyrefractory assemblies having a temperature of use of generally up to950° C., even 980° C., all the reactive brazing compositions chieflycontaining silver, or silver-copper, copper, nickel, iron or cobalt,platinum, palladium or gold are therefore to be excluded on account oftheir strong reactivity with silicon carbide.

Formulations of brazing alloys, brazing compositions, that are morerefractory and with high silicon content are presented in documents [1,2, 3]. These brazing compositions have scarcely reactive behaviour, evennon-reactive, with SiC which prevents the formation of fragilecompounds. This criterion of non-reactivity or very low reactivity isnot a sufficient condition however for guaranteeing good mechanicalstrength of the brazed joints. In the literature, the yield strengthvalues of binary silicon-based brazing alloys are most variable inrelation to the second element taking part in the silicon-basednon-reactive brazing composition.

For example, for the non-reactive Fe—Si system (45% Fe-55% Si byweight), document [3] mentions an extremely low ultimate tensilestrength of the order of 2 MPa, despite the non-reactivity of thiscomposition indicated in document [4], whilst for the Cr—Si system (25%Cr-75% Si by weight), this same document [3] gives a higher value of theorder of 12 MPa.

For a non-reactive Co—Si alloy (90% Si-10% Co by weight), document [1]mentions a value of about 100 MPa under compression/shear.

The properties, in particular mechanical properties, of a silicon-basedbrazing composition are fully unpredictable and can absolutely not beinferred from the mechanical properties of already known Si-basedbrazing compositions, even if of very close type.

In other words, when it is sought to prepare a silicon-based brazingcomposition in particular for brazing parts in SiC, it is absolutely notpossible to refer to the mechanical properties which may be acceptableexhibited by other known Si-based brazing compositions, since anymodification however small of a Si-based brazing composition whetherconcerning the type of the metal(s) brazed with the silicon or theproportions thereof, may lead to unpredictable, unexpected even majorchanges in the properties of the composition and in particular itsmechanical properties.

To conclude, it is not possible to predict the mechanics of a givenbinary Si—X system where X is a metal, and even less so the mechanics ofa said system as a function of the proportions of X. For all the morereason, it is impossible to predict the mechanics of a more complexsystem such as a ternary Si—X—Z system where X and Z are metals.

The brazing temperatures of the brazing compositions in documents [1, 2]and [3] are generally higher than 1300° C. These brazing temperature arefor example 1355° C. for the Ti—Si composition (22-78% by weight), 1355°C. for the Cr—Si composition (25-75% by weight), 1400° C. to 1450° C.for the Co—Si composition, and 1750° C. for the Ru₂Si₃ composition.

The efficacy of this joining method requires brazing temperatures higherthan 1300° C. for thermodynamic destabilization of the passivatingsilicon oxide layers which occur spontaneously on the silicon carbidesurfaces, since these silicon oxide layers are detrimental to wetting bythe brazing composition, even if brazing is conducted in a vacuum.

Therefore the above-mentioned brazing alloys with high silicon contentand used at a temperature higher than 1300° C. are not suitable for thebrazing of substrates in silicon carbide-based materials whoseproperties are degraded after exposure to 1300° C., even more so forthose which degrade at 1150° C., even 1100° C. or lower. This is notablythe case with some SiC/SiC composites which degrade at above 1300° C.,even 1150° C., and even at above 1100° C. such as the CMC examined inthe Examples which degrades on and after 1100° C.

It is true that document [3] in Example 2 presents a Ni—Si brazingcomposition (65% Ni-35 Si % by weight, i.e. 47 atomic % Ni-53 atomic %Si) which can be brazed at 1120° C., for 16 hours. This brazingtemperature is slightly higher than the preferred brazing temperatureused in the invention which is 1100° C., but it uses a very long brazinghold time. However, the mechanical strength of the joint obtained withthis composition (ultimate tensile strength of 375 p.s.i.—i.e. about 2.6MPa) is very low despite the non-reactivity of this compositionmentioned in document [5]. This mechanical strength is insufficient fornumerous applications and in particular the main applications concernedherein, despite the low reactivity of this brazing composition with SiC.

It is also to be pointed out that this Ni—Si brazing alloy (65 wt %-35wt %) has a melt onset temperature of 966° C. (eutectic at 966°) whichis not suitable for applications at 950° C.-980° C.

For higher Si contents, it is specified in document [5] that Ni—Sibrazing alloys are not reactive, but no mechanical data is provided. Thework described in document [5] focuses on the study of wetting anglesand the work of adhesion (thermodynamic adhesion at a solid/liquidinterface, this adhesion is defined by the work needed for reversibleseparation of a solid/liquid interface into two solid/vapour andliquid/vapour surfaces. Finally, it is noted that for these Ni—Sibrazing alloys, the range between liquidus and solidus is very extensivewith, as already mentioned above, the onset of melting on and after 966°C. (for Ni 66% by weight, but also for close-lying contents due to thepresence of an eutectic at 966° C.) which limits applicationtemperatures to below 900° C.

Document [6] mentions a brazing alloy Ni-13.4 Cr-40 Si (atomic %) whosemelting point is 1150° C. and which is used at a brazing temperature of1200° C. The authors did not conduct mechanical characterization on thebrazed joints and only metallurgical characteristics are given whichindicate non-reactivity.

No mechanical test result on this alloy is provided which means thatgood mechanical strength of the brazing can in no way be guaranteed.

Document [2] proposes (Example 3) a Pt—Si alloy which is brazed at 1200°C. The Pt content of this brazing composition is very high (77 weight %Pt), which leads to a very costly process. This disadvantage isprohibitive for the obtaining of large-size brazed parts.

At all events, 1200° C. is a temperature that is too <<refractory>> forthe applications concerned by the present invention.

Finally document [7] presents brazing alloys having a Si content of lessthan 50 weight %, preferably 10 to 45 weight %, and with the addition ofat least 2 elements chosen from the following group: Li, Be, B, Na, Mg,P, Sc, Ti, V, Cr, Mn, Fe, Co, Zn, Ga, Ge, As, Rb, Y, Sb, Te, Cs, Pr, Nd,Ta, W and Ti. In this group of elements at least one thereof ispreferably a metal chosen from among Fe, Cr, Co, V, Zn, Ti and Y.Neither nickel nor aluminium are cited.

The examples in document [7] describe ternary brazing compositions:Si—Cr—Co (11:38.5:50.5% by weight); Si—Cr—Co (40:26:34% by weight);Si—Fe—Cr (17.2:17.5:65.3% by weight); and Si—Fe—Co (20:20:60% byweight); and the brazing thereof at temperatures respectively of 1230°C., 1235° C., 1460° C. and 1500° C.

The brazing compositions in document [7] never contain the nickelelement or aluminium element.

Regarding the brazing compositions having brazing temperatures lowerthan 1300° C., it is simply mentioned that a <<strong>> bond is obtainedand no mechanical test is provided to prove that good mechanicalstrength of the joints is effectively obtained. Also, the low reactivityof the SiC/brazing filler is neither mentioned nor referred to.

In the light of the foregoing there is therefore a need, not yet met,for a method with which it is possible to obtain the joining by brazingof parts in silicon carbide-based materials, more specifically ofmoderately refractory substrates in silicon carbide, which ensuressatisfactory mechanical strength of the assembly at between 20° C. and950° C. even 980° C., in particular above 500° C. and up to 950° C. even980° C., and optionally also sealing of the joint.

This method must allow the use in particular of brazing temperaturesequal to or lower than 1150° C. and preferably of 1100° C. which is atemperature that it is absolutely essential not to exceed for someSiC-based substrates, parts to be joined.

It is effectively essential that the parts, substrates maintain theirfull integrity and initial performance levels after the joiningoperation by brazing.

There is therefore a need for a brazing method using brazingcompositions which allows the desired temperatures of use to be reachednamely up to 950° C. even 980° C., whilst avoiding the subjecting of theparts, substrates in silicon carbide-based materials to temperatureranges which could deteriorate these materials.

In other words, there is a need for a brazing method which allowsmoderately refractory brazed joints to be obtained (with a temperatureof use of up to about 950° C. even 980° C.) using brazing cyclesgenerally not exceeding a limit temperature defined between 1040° C. and1150° C., in particular of 1100° C. depending on the SiC-based materialto be joined.

Numerous silicon carbide-based materials, in particular some composites,are irreversibly deteriorated over and above 1100° C.: this isparticularly the case with some composites formed of a SiC matrix andSiC fibres such as the composite available from SNECMA Propulsion Solideunder the trade name Cerasep A40C®.

In addition, the holding time of the brazing plateau at a temperatureequal to or lower than 1150° C., for example of 1100° C., mustpreferably be from one or a few minutes to two or three hours at most toavoid degradation of the composite.

On the other hand, pure silicon carbide withstands brazing at 1450° C.

In other words, there is a need for a brazing method and composition,brazing alloy, firstly allowing the use of the full refractory potentialof the silicon carbide-based substrates at temperatures of use of up toabout 950° C. even 980° C., and secondly allowing brazing at a brazingtemperature lower than the degradation temperature of the substrateswith a brazing temperature equal to or lower than 1150° C., preferablyin the range between 1040° C. and 1150° C., more preferably lower than1100° C., further preferably in the range between 1080° C. and 1100° C.

There is also a need for a method allowing brazing to be conducted at atemperature equal to or lower than 1150° C., preferably between 1040° C.and 1150° C., of a moderately refractory assembly (temperature of usegenerally between 950° C. and 980° C.), of parts in siliconcarbide-based materials irrespective of their shape and/or their size.

In particular, there is a need for a brazing method and for theassociated brazing composition, allowing the brazing to be conducted ata temperature lower than 1150° C., preferably between 1040° C. and 1150°C., of silicon carbide-based parts of large size and/or of complexgeometry notably having large surface areas to be brazed.

In addition, none of the methods and compositions in the prior artsimultaneously meets the following criteria evidenced by the inventorswhich are fundamental for preparing structural components in SiCentailing moderately refractory joints:

1) the brazing composition must allow a strong bond to be obtainedbetween the two parts in silicon carbide-based material, whichnecessitates a non-reactive brazing composition i.e. chemicallycompatible with silicon carbide, and which does not form fragilecompounds therewith. However, the non-reactivity does not guarantee theforming of a strong bond since this remains unpredictable.Non-reactivity is a condition for obtaining a strong bond but it is notsufficient. For example, the Fe—Si system cited in the literature [3] isnon-reactive but its mechanical strength is very weak;

2) the brazing composition must obtain good wetting of the siliconcarbide and good adhesion thereto;

3) the brazing composition must be compatible with all heating devicesin particular rapid and/or localised heating devices;

4) the brazing composition must allow the formation of joints havinggood mechanical strength;

5) the brazing composition must be formed of a limited number ofelements to facilitate the preparation and implementation thereof;

6) the brazing composition must not contain costly elements such asprecious metals.

Finally, the method and associated brazing composition must allow thebrazing, the joining of any type of silicon carbide-based material, andmust be easily adaptable to any specific silicon carbon-based ceramic.

The objective of the invention is therefore to provide a method for thejoining by brazing of parts or components in silicon carbide-basedmaterials which inter alia meets the above-cited needs, which inter aliafulfils all the requirements and criteria set forth above, whicheliminates the disadvantages, defects, limitations encountered withprior art methods and which solves the problems of the prior artmethods.

The objective of the invention is notably to provide a method for thejoining by brazing of parts or components in silicon carbide-basedmaterials which allows satisfactory mechanical strength of the assemblyto be obtained above 500° C. and up to 950° C., even 980° C., which usesbrazing temperatures equal to or lower than 1150° C., preferably in therange between 1040° C. and 1150° C., and more preferably equal to orlower than 1100° C., for example from 1080° C. to 1100° C., and whichoptionally allows the obtaining of joints having an excellent seal.

DISCLOSURE OF CERTAIN INVENTIVE ASPECTS

This goal and others are achieved according to the invention by a methodfor joining, assembling, at least two parts made of siliconcarbide-based materials by non-reactive brazing, wherein the parts arecontacted with a non-reactive brazing composition, the assembly formedby the parts and the brazing composition is heated to a brazingtemperature sufficient to melt the brazing composition totally or atleast partly, and the parts and the brazing composition are cooled sothat, after solidification of the brazing composition, a moderatelyrefractory joint is formed; wherein the non-reactive brazing compositionis an alloy comprising in atomic percentages 45% to 65% silicon, 28% to45% nickel, and 5% to 15% aluminium.

Advantageously, the alloy is a ternary alloy consisting, in atomicpercentages, of 45% to 65% silicon, 28% to 45% nickel, and 5% to 15%aluminium.

The melting of the brazing composition is generally considered to betotal when it is in the liquid state at a temperature equal to or higherthan the liquidus. The melting of the brazing composition is generallyconsidered to be partial when it is in a state that can be qualified assemi-solid, viscous, softened, at a temperature lying between thesolidus and the liquidus.

In general, brazing is conducted at a brazing temperature equal to orlower than 1150° C., preferably the brazing temperature is from 1040° C.to 1150° C., more preferably from 1080° C. to 1100° C.

By moderately refractory joint is generally meant that this joint isgenerally capable of withstanding operating temperatures of up to 950°C. even 980° C.

The method of the invention, which is a brazing method at a temperaturefrom 1040° C. to 1150° C., more preferably from 1080° C. to 1100° C. andusing a specific brazing composition, has never been described in theprior art.

In particular, the specific brazing composition used according to theinvention which surprisingly allows the brazing at a temperature from1040° C. to 1150° C., preferably from 1080° C. to 1100° C., of partsmade of silicon carbide-based materials is in no way mentioned in theabove-cited prior art documents.

For example, document [7] does not mention either nickel or aluminium inthe list of 27 elements among which at least two must be chosen to forma brazing composition with silicon, silicon at all times being presentto a proportion of less than 50% by mass. In this list Fe, Cr, Co, V,Zn, Ti and Y are preferred, and for all the more reason neither nickelnor aluminium are cited among the preferred elements.

None of the particularly preferred brazing alloys of this document whichare alloys of silicon, chromium and cobalt, alloys of silicon, chromiumand iron, and alloys of silicon, iron and cobalt, and none of thebrazing compositions exemplified in this document contain eitheraluminium or nickel.

The brazing compositions in the Examples given in document [7] arelimited to ternary SiFeCo, SiFeCr, SiCrCo systems having a Si content ofless than 40% by mass. These compositions contain neither aluminium nornickel and globally contain a much reduced number of elements comparedwith the list of 27 possible addition elements.

Document [7] does not provide any indication possibly leading to thechoice of aluminium and nickel, and moreover no indication regarding aspecific content thereof for preparing a ternary braze alloy compatiblewith SiC and ensuring the brazing of SiC-based parts at a temperaturefrom 1040° C. to 1150° C., preferably from 1080° C. to 1100° C., and theeffective joining of these parts.

The method of the invention fulfils the needs, meets all therequirements and criteria mentioned above and does not have thedisadvantages of the prior art methods.

In particular, for the first time, the method of the invention allowsthe preparation of moderately refractory assemblies i.e. having atemperature of use of up to 950° C., even 980° C., of parts made ofsilicon carbide-based materials irrespective of their geometry, evenvery complex, and/or their size.

The method of the invention in all cases particularly ensures goodfilling of the joint with the brazing composition, excellent mechanicalstrength of the assembly at ambient and hot temperature in particularabove 500° C. and up to 950° C.-980° C., and optionally very goodimperviousness, leak tightness of the joint.

The method of the invention is additionally simple, reliable, easy toimplement and overall of low cost.

In other words, the multiple advantages and surprising effects of theinvention can be enumerated as follows, this enumeration not to beconstrued as limiting:

-   -   in relation to the composition of the chosen brazing alloy,        several brazing temperatures are possible between 1040° C. and        1150° C. and are therefore able to meet different        specifications;    -   the assembly obtained with the invention allows to ensure a good        mechanical adhesion between the silicon carbide-based substrates        even at maximum temperatures of use of more than 500° C. and        possibly reaching 950° C., even 980° C. for example. Ruptures        occur in “cohesive” mode i.e. cracks occur in the silicon        carbide substrates and not at the brazed joint;    -   the brazing temperature is equal to or lower than 1150° C.,        preferably between 1080° C. and 1100° C.; it is therefore        possible with the method of the invention to join silicon        carbide-based parts, substrates which cannot withstand        temperatures of more than 1150° C., such as composite parts,        substrates with a ceramic matrix e.g. Cerasep A40C®. In other        words, with the method of the invention it is possible perform        the brazing of SiC-based parts which degrade, deteriorate, on        and after 1150° C., even 1100° C. Evidently, the method of the        invention applies to pure or near-pure SiC, for example sintered        SiC (in general sintered SiC contains sintering additives and is        therefore not perfectly pure) for which brazing temperatures        higher than 1300° C. can be used, but it also applies to less        stable materials using brazing compositions adapted to these        less heat-stable materials;    -   surprisingly, despite the brazing temperature equal to or lower        than 1150° C., preferably from 1040° C. to 1150° C., more        preferably from 1080 to 1100° C. used in the method of the        invention, excellent wetting of the brazing composition, of the        braze alloy of the invention on the surfaces of the silicon        carbide substrates, parts to be joined has been ascertained.        Therefore, by means of this good wetting of the surfaces it is        possible according to the invention to conduct capillary brazing        since the brazing composition of the invention is capable alone        of filling the joint between the parts during the brazing        operation for joints of a few microns to a few tens of microns,        but also for thicker joints whose thickness may reach 500 μm.        Capillary brazing with reinforcements e.g. particles or fibres        in the joint, allows joints to be produced having a thickness of        more than 500 μm and possibly even reaching a few millimetres;    -   non-reactivity of the braze alloy with the silicon carbide-based        substrates was observed on the scale of scanning electron        microscopy. There are no complex, porous weakening zones at the        interface;    -   the brazing obtained with the method of the invention is        reversible. It is therefore possible to disjoin, separate the        assembled parts, substrates for example for the repair thereof        by melting the brazing alloy in a furnace during a second        melting operation of this braze alloy, without deteriorating the        parts, substrates. The parts, substrates can also be separated        by chemical attack. In other words, the method of the invention        allows the repair of joined parts made of silicon carbide        material. This means that these parts can be subjected to a        second brazing cycle if needed for the purpose of repair without        deteriorating the properties of the joints. This capacity for        repair is possible due to the non-reactivity or scarce        reactivity of the braze alloys used in the invention with        silicon carbide;    -   another remarkable property obtained with the method of the        invention is the homogeneity of the joint obtained after brazing        and the very good mechanical behaviour of the joints formed;    -   it is not necessary in the method of the invention to metallize        the parts, substrates made of SiC materials with the brazing        composition before the brazing operation at a temperature equal        to or lower than 1150° C., since the joints are well filled with        the brazing composition of the invention, even in capillary        configuration;    -   it is further not necessary in the method of the invention to        deposit carbon on the parts, substrates made of SiC-based        materials before the brazing operation at a temperature in        particular lower than 1150° C. The wetting kinematics are rapid        and the wetting angle is very good (a wetting angle of 50° is        obtained after 5 minutes at 1100° C. and an angle of 30° is        obtained after 30 minutes at 1100° C.—cf. Examples 2 and 1) and        the joints are well filled with the brazing composition of the        invention, even in capillary configuration;    -   the brazing compositions of the invention do not contain any        precious chemical element, in particular no metals from the        platinum or rhodium family, which limits their cost and the cost        of the method in which they are used compared with numerous        prior art compositions;    -   the brazed joints obtained with the method of the invention are        generally impervious, leak tight. The method of the invention is        therefore adapted for sealing operations which must withstand        maximum temperatures of between 950° C. and 980° C. depending on        the brazing alloy composition.

As already mentioned above, the behaviour of the brazing compositions,more particularly for brazing SiC, is extremely unpredictable and cannotunder any circumstance be inferred from the behaviour of like brazingcompositions.

Advantageously, the brazing composition of the invention may comprise,preferably be composed (consist) of, 60% to 55% silicon, 30% to 34%nickel and 11% to 9% aluminium in atomic percentages.

The preferred composition of the invention comprises, preferably iscomposed (consists) of, 57.5±1% silicon, 32.5±1% Ni and 10±0.5% Al, inatomic percentages.

This preferred composition has a solidus temperature of 1030° C. and aliquidus temperature of 1060° C.

The different brazing compositions defined by the advantageouspercentages specified above are neither described nor suggested in theprior art.

The wetting of SiC-based substrates by the SiNiAl alloy (in particularfor compositions in the preferred range between 60 and 55 atomic % Si,30 and 34 atomic % Ni and 11 and 9 atomic % Al) is good since, asalready indicated above, wetting angles of the order of 50° and 30° arerespectively obtained after 5 minutes and 30 minutes at 1100° C. in agraphite furnace (cf. Examples 1 and 2). There is no need to depositcarbon on the silicon carbide in order to obtain a small stationaryangle of the order of 30°.

Compression/shear tests were conducted on SiC/NiSiAl/SiC joints (cf.Example 3) and the breaking stress values were high with an average of48 MPa (for the five test pieces the values obtained were the following:33 MPa, 67 MPa, 35 MPa, 53 MPa and 54 MPa). Breaking occurred in the SiCwhich is characteristic of strong bonds between the braze alloy and thesubstrate.

Advantageously, prior to contacting the parts with the brazingcomposition, an addition, supply of a a reinforcement is carried out, inthe brazing composition, and/or on at least one of the surfaces to bejoined, assembled, of at least one of the parts to be assembled, joined,and/or in the vicinity of at least one of the surfaces to be joined,assembled, of at least one of the parts to be assembled, joined, and/orbetween the surfaces to be joined, assembled, of the parts to beassembled, joined.

This reinforcement may be made of a material chosen from among ceramicssuch as SiC and C.

This reinforcement may be in the form of particle e.g. of a powder; offibres; of non-woven fabric; of woven fabric; of a felt; of a foam.

The adding of the reinforcement may be conducted in an amount of 50% byvolume at the most, preferably from 1 to 49% by volume, more preferably5 to 49% by volume, relative to the volume of the brazing composition.

Advantageously, when the reinforcement is in the form of particles orfibres, these particles or these fibres may be placed in suspension inan organic binder to obtain a suspension, slurry, or paste ofreinforcing particles or fibres, and at least one surface to be joined,assembled, of at least one of the parts to be assembled, joined, may becoated with the suspension, slurry, or paste of reinforcing particles orfibres.

Advantageously, prior to the addition, supply of the reinforcement inthe brazing composition and/or on at least one of the surfaces to bejoined, assembled, of at least one of the parts to be assembled, joined,the reinforcement is optionally subjected to a heat treatment at atemperature generally from 1300° C. to 1500° C. e.g. 1400° C., for atime generally from 2 to 4 hours, for example 3 hours, under a highvacuum, then the reinforcement is optionally stored in an inertatmosphere e.g. in an argon atmosphere, for example if it is not used onthe same day.

This heat treatment notably applies to SiC reinforcements, in particularin fibres or particles form, since SiC oxidizes at ambient temperaturewith the formation of silica, which is not the case with carbonreinforcements.

More generally, this heat treatment may prove to be necessary when thereinforcements to be used, in particular in powder form, are highlyoxidized.

Advantageously in the method of the invention it is possible to form abrazing composition powder, to place this powder in suspension in anorganic binder so as to obtain a suspension, slurry or paste of brazingcomposition, and to coat at least one surface of at least one of theparts to be joined, assembled, with the suspension, slurry, or paste ofbrazing composition obtained.

For example, it is possible to coat at least one surface to be joined,assembled, of at least one of the parts to be assembled, joined, withthe suspension, slurry or paste of brazing composition, then to place incontact the surfaces to be assembled, joined, of the parts to be joined,assembled, so that the suspension, slurry, or paste of brazingcomposition is inserted between these surfaces.

Or else it is possible to place in contact the surfaces to be joined,assembled, of the parts to be joined, assembled by leaving an offsetbetween them so as to create a free surface able to receive thesuspension or paste of brazing composition in the vicinity of the jointformed by the surfaces to be joined, assembled, of the parts to beassembled, joined, then the suspension or paste of brazing compositionmay be deposited on this free surface for example in the form of a bead.

In this latter embodiment, the joint formed by the surfaces to bejoined, assembled, of the parts to be joined, assembled canadvantageously be occupied by a reinforcement which also preferablycovers said free surface and on which the suspension or paste of brazingcomposition is deposited.

Prior to contacting with the brazing composition, the depositing ofcarbon on at least one of the surfaces of the parts to be assembled isnot necessary.

This is precisely another advantage of the method according to theinvention in that this carbon deposit can be omitted, thereby avoidingan additional step in the brazing method.

Advantageously, the brazing can be conducted at a brazing temperaturethat is at least 15° C. higher, preferably at least 30° C. higher thanthe melting point of the brazing composition.

For the brazing of porous brazing surfaces, for example for compositematerials whose SiC surface coating is insufficiently thick, it may beuseful to conduct brazing at a temperature between the liquidus and thesolidus of the brazing composition to obtain a brazing composition inthe semi-solid state during the brazing (temperature) plateau. The brazecomposition is then viscous and the infiltration thereof into theporosities can be better controlled.

Advantageously, brazing can be performed by conducting a brazing plateauat a brazing temperature from 1040° C. to 1150° C., preferably 1080° C.to 1100° C., held for a time of 1 to 150 minutes, preferably 30 to 150minutes, more preferably 60 to 120 minutes, further preferably 90 to 120minutes.

If at least one surface to be joined, assembled, of the parts to beassembled, joined, is porous, a brazing temperature plateau at 1040° C.to 1100° C. held for a time of 1 to 30 minutes can be applied.

In other words, for materials having surfaces to be brazed that arerelatively porous, such as composite materials whose SiC coating is ofinsufficient thickness, it may be useful to reduce the usual brazingtime which is generally of the order of 30 to 150 minutes, to a time ofa few minutes namely a time of between 1 and 30 minutes for example, toavoid too much infiltration of the brazing composition into theporosities of the material to the detriment of joint filling. In thiscase, it is also to be noted that the lowest brazing temperatures aregenerally recommended to limit infiltration, namely between 1040° C. and1100° C.

Advantageously, prior to the brazing (temperature) plateau, it ispossible to observe a first plateau at a temperature generally from 950°C. to 1000° C., preferably 980° C., generally held for a time of 30 to180 minutes, preferably 60 to 180 minutes, more preferably 90 to 180minutes, for example 120 minutes.

Advantageously, the silicon carbide-based materials may be chosen fromamong pure silicon carbides such as pure α silicon carbide (α-SiC) orpure β silicon carbide (β-SiC), and from SiC-based composite materialssuch as composites with silicon carbide fibres and/or matrix.

More particularly, the silicon carbide-based materials may be chosenfrom among pressureless sintered silicon carbide (“PLS-SiC”);Si-infiltrated silicon carbide (“SiSiC” or “RBSC”); porousrecrystallized silicon carbide (“RSiC”); graphite silicon (“C-SiC”)composed of graphite coated with a SiC layer; SiC/SiC composites, forexample with fibres or whiskers; SiC/SiC composites with self-healingmatrix; C/SiC composites, for example with carbon fibres or whiskers andSiC matrix; SiC monocrystals; SiC composites with another ceramic forexample SiC/Si₃N₄ and SiC/TiN composites.

In general, the said silicon carbide-based materials have a siliconcarbide content of at least 50% by mass, preferably at least 80% by massand more preferably of 100% by mass.

The invention also concerns the brazing compositions described above inthe description of the method according to the invention.

The invention also pertains to a composition for the brazing, forexample the non-reactive, moderately refractory brazing of parts made ofsilicon carbide-based materials, comprising a non-reactive brazingcomposition such as defined above and also comprising an addition,supply of a reinforcement such as defined above.

The invention further pertains to a brazing paste, slurry or suspensionfor the brazing, for example the non-reactive moderately refractorybrazing of parts made of silicon carbide-based materials, comprising apowder of a brazing composition according to the invention such asdefined above or of a composition according to the invention for thenon-reactive brazing of parts such as defined above, and an organicliquid cement, binder, or an organic viscous gel.

The invention also concerns the moderately refractory joint (maximumtemperature of use generally of between 950° C. and 980° C.), and theassembly comprising at least two parts made of SiC-based materialsobtained using the method of the invention described above.

Other characteristics and advantages of the invention will become betterapparent on reading the following description given as a non-limitingillustration and with reference to the appended drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the arrangement of the plates ofSiC-based material and of the paste of brazing composition for brazingin <<sandwich>> configuration.

FIG. 2 is a schematic view showing the arrangement of the plates ofSiC-based material and of the paste of brazing composition for brazingin capillary configuration.

FIG. 3 is a graph illustrating the brazing heat cycle recommended forbrazing a joint with a reinforcement of SiC particles or SiC fibreswhich is also the brazing heat cycle used in Example 6. The time inminutes as from the start of heat treatment is given along the X-axis,and the temperature T in ° C. is given along the Y-axis.

FIG. 4 is a schematic view illustrating the arrangement of the plates ofSiC-based material and of the paste of brazing composition for thebrazing in capillary configuration, such as especially conducted inparticular in the example of a joint with a reinforcement of SiCparticles or SiC fibres emerging from the joint.

FIG. 5 is a graph illustrating the brazing heat cycle used in Example 3.The time in minutes as from the start of heat treatment is given alongthe X-axis, and the temperature T in ° C. is given along the Y-axis.

FIG. 6 is a schematic view of the test pieces used for mechanicaltesting, in particular compression/shear testing of the joints and theassemblies prepared in the Examples.

FIG. 7 is a schematic view illustrating the arrangement of the plates ofSiC-based material and of the paste of brazing composition for thebrazing in capillary configuration, such as conducted in Example 5, of ajoint with a reinforcement of SiC particles or SiC fibres emerging fromthe joint.

DETAILED DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

The first step of the method according to the invention generallyconsists of preparing, forming a braze, brazing composition, in otherwords a braze, brazing, alloy containing silicon, nickel and aluminium.

The braze, brazing, alloy of the invention is a ternary Silicon(Si)-Nickel (Ni)-Aluminium (Al) alloy.

The melting point of the braze alloy of the invention is generally from1060° C. (solidus at 1030° C. and liquidus at 1060° C.). The predominantelement of the alloy is silicon.

The mass proportions of the ternary Si—Ni—Al alloy, in atomicpercentages, are 45% to 65% silicon, 28% to 45% nickel and 5% to 15%aluminium.

Advantageous proportions and particularly advantageous or preferredproportions were indicated in the foregoing.

The brazing composition is generally a powder composition which can beprepared, for example, by first synthesizing, from pure Si, Ni and Alelements, an intermetallic compound containing silicon, nickel andaluminium.

The synthesis of said intermetallic compound is carried out, forexample, by adding silicon (e.g. in the form of pieces), nickel (e.g. inthe form of pieces or other forms) and aluminium (e.g. in the form ofpieces or other forms) in the desired proportions for the brazingcomposition, in a refractory crucible made of alumina, for example.

The assembly formed by the crucible, silicon, nickel and aluminium isplaced in an heating apparatus such as a graphite furnace and is heatedto a temperature generally of 1300° C., preferably under a high vacuum,for a time of 30 minutes for example, to melt the different constituentsof the brazing composition and to obtain, after cooling, the finaldesired intermetallic compound that is homogeneous and in ingot form.The heating temperature is preferably 1300° C. for the preferredcomposition of the invention.

The fabrication of the ingot can also be carried out in a cold crucible.This contactless melting technique (copper crucible cooled bycirculating water and placed in an inductor) allows the alloy to bemelted without contacting the crucible and hence the recovery thereofwithout machining the crucible.

The ingot of intermetallic compound obtained is then ground using anysuitable apparatus for example in a mortar to obtain a powder ofadequate particle size, i.e. whose particles have a diameter of 1 to 300μm for example, and which constitutes the brazing composition.

The second step of the method of the invention generally entails theactual joining, assembling, by brazing.

Prior to assembling, joining, the two (or more) surfaces of the partsmade of SiC materials to be joined are generally degreased, cleaned inan organic solvent for example of ketone, ester, ether, alcohol type, ora mixture thereof, etc.

One preferred solvent is acetone or an acetone-ethyl alcohol-ethermixture for example in proportions of 1:3, 1:3, 1:3; it is also possibleto clean the parts successively with several different solvents, forexample with acetone followed by ethanol. The parts are then dried.

The parts made of SiC-based materials to be assembled are generally twoin number, but it is also possible to join simultaneously a largernumber of parts of possibly up to 100.

By part made of SiC-based material is generally meant any element orentity of any shape and size which, after assembly with one or moreother parts, enters into structures of larger size.

According to the invention it is possible, each time with excellentresults, to join parts of complex geometry, shape, and/or of large sizefor example having a surface area of 0.5 m² or more to be brazed.

By silicon carbide-based material is generally meant herein anymaterials containing at least 50% by mass of silicon carbide, preferablyat least 80% by mass of silicon carbide, further preferably 100% by massof silicon carbide; in this latter case, the material consists, iscomposed solely of silicon carbide.

The silicon carbide-based materials may notably be in the form of asintered or infiltrated powder or of fibres bound by a ceramic matrix.

The silicon carbide-based materials may be chosen from among puresilicon carbides such as pure α silicon carbide (α-SiC) or pure βsilicon carbide (β-SiC) and SiC-based composite materials such ascomposites with silicon carbide fibres and/or matrix.

As examples of SiC-based materials, mention may be made of pure densesilicon carbide or pressureless sintered silicon carbide (PLS-SiC); Siinfiltrated silicon carbide (SiSiC or RBSC containing 5 to 20% Si);porous recrystallized silicon carbide (RSiC); graphite silicon (C-SiC)composed of graphite coated with a layer of SiC for example to athickness of 0.1 to 1 mm; and SiC/SiC composites, for example withfibres or whiskers; SiC/SiC composites with self-healing matrix; C/SiCcomposites, for example with carbon fibres or whiskers and a SiC matrix;and also SiC monocrystals; SiC composites with another ceramic, forexample SiC/Si₃N₄ and SiC/TiN composites.

Preferably, the silicon-based material of the substrates, parts to bejoined according to the invention is composed of 100% silicon carbidechosen for example from among sintered pure α silicon carbide (α-SiC) orpure β silicon carbide (β-SiC) or from among composites with siliconcarbide fibres and a silicon carbide matrix.

It has surprisingly been ascertained that the method of the inventionallows the brazing of composites with excellent results (cf. Examples 4to 6).

The two or more parts to be joined may be made of one same siliconcarbide-based material for example of PLS (<<PressurelessSintered>>)-SiC, or of a SiC-SiC composite, or each of the parts may bemade of a different silicon carbide-based material.

The suspension, paste of the brazing composition prepared as describedpreviously is spread, coated, applied homogeneously, uniformly, usingfor example a brush or spatula or a syringe optionally fixed to arobotized system, or using any other means allowing a uniform layer ofbrazing paste to be deposited on the surface of at least one of theparts made of silicon carbide-based material to be joined.

The paste-coated surface(s) of the two parts (1, 2) to be joined arethen placed in contact. This brazing configuration illustrated in FIG. 1is called a <<sandwich configuration>> since the paste of brazingcomposition (3) is placed directly between the surfaces (4, 5) of theparts to be joined.

Preferably in this <<sandwich>> configuration it is recommended, for thebrazing composition of the invention, that the brazing paste should notbe uniformly distributed but applied in the form of beads of brazingcomposition, braze alloy, which do not touch each other to avoid aconfiguration that is too confined.

The quantity of paste, suspension of brazing composition to be used inthis configuration is generally 10 mg/cm² to 50 mg/cm², for example 20mg/cm².

The <<sandwich>> configuration applies both to <<thin>> joints i.e.having a thickness of less than 500 micrometres, and to <<thick>> jointsi.e. having a thickness of 500 micrometres or more.

Or else, as is illustrated in FIG. 2, the parts to be joined, forexample in the form of plates (21, 22), are placed in contact withouthaving put the brazing composition between them but by leaving a gap, anoffset (23) therebetween generally of a few mm, for example of 1 mm, 2mm, to 10 mm so as to create a free surface (24) able to receive thesuspension or paste in the vicinity of the joint (25) formed by thesurfaces to be joined, assembled, of the parts to be assembled, joined,then the suspension or paste of brazing composition is deposited forexample in the form of a bead of brazing composition, braze alloy, (26)on this surface in the vicinity of the joint or on the edge of thejoint. During the brazing heat cycle, the liquid brazing compositioninfiltrates into the joint.

This brazing configuration is called a <<capillary configuration>>. Withthe brazing compositions of the invention it is possible to conduct saidcapillary brazing, with infiltration of the liquid brazing composition,braze alloy into the brazed joint during the brazing cycle, withoutdepositing the brazing composition directly between the parts to beassembled as in the <<sandwich>> configuration.

This capillary configuration is even preferred for the Ni—Si—Al systemsince it allows better filling of the joint to be obtained with thebrazing composition, braze.

The quantity of paste, suspension of brazing composition to be used inthis capillary configuration is generally 10 mg/cm² to 30 mg/cm², forexample 20 mg/cm².

Capillary brazing is possible for <<thin>> joints having a thickness ofless than 500 μm, without reinforcements previously placed in the joint.Capillary brazing led to a good filling of the joints by the Ni—Si—Albraze alloy, the joint thickness possibly varying from a few microns toalmost 500 μm for parts made of SiC/SiC composite having surfacedefects.

Capillary brazing is also possible for joint thicknesses much higherthan 500 μm possibly reaching a few millimetres, for joints in which a<<wetting reinforcement>> (i.e. the braze alloy provides good wetting ofthe surface of the reinforcement, this being the case with SiC-basedreinforcements for example) by the brazing composition has been placedbetween the surfaces to be brazed.

This reinforcement may be in the form of ceramic particles for exampleof a ceramic powder such as SiC, of fibres for example of ceramic fibressuch as SiC, or C particles, of SiC fibres, of woven fibres for exampleof SiC, of non-woven fibres; of a felt or of a foam. The SiC powder maybe a commercial powder for example such as the powder of the trade nameSTARCK® having a purity of 98.5% and a particle size of less than 10 μm,or the powder of trade name Neyco® having a purity of 98.5% and aparticle size of 50 μm. For thicknesses higher than 500 μm, the qualityof the joint is better with reinforcements of SiC particles or of SiCfibres which reduce cracks in the joint.

Regarding the nature of the reinforcement and the implementationthereof, reference can be made to the corresponding pages in thedescription of document [2].

The braze alloy placed on the edge of the joint changes to the liquidstate during the brazing cycle, infiltrates the joint and wets thereinforcements which allows a joint to be obtained that is well filledwith the braze alloy.

The reinforcements therefore allow infiltration into thick joints.

It was evidenced, in accordance with the invention, that theinfiltration of the braze alloy into the joint and the wetting of thereinforcements are possible and occur optimally under certainconditions.

In other words, it was evidenced that to obtain good filling without anyvacancies of braze alloy in particular in the centre of the joint,several specific steps had to be heeded.

These specific steps are the following:

-   -   first, optionally heat treatment of the reinforcement at a        temperature generally from 1300° C. to 1500° C. e.g. 1400° C.,        under a high vacuum in a graphite furnace generally for 2 to 4        hours, before use;    -   after heat treatment of the reinforcement, if it is not to be        used immediately, it must be stored preferably under argon;    -   the brazing cycle must necessarily comprise 2 plateaux as        described below (see FIG. 3):        -   a first plateau at a temperature of 950° C. to 1000° C., for            example 980° C., for a time of 2 to 4 hours, for example of            3 hours;        -   followed by a second plateau which is the brazing plateau            such as described below and which is conducted in particular            at a temperature of 1080° C. to 1100° C. for 90 to 150            minutes, for example at a temperature of 1100° C. for 90 or            120 minutes to fill typically a joint length of 3 cm of a            joint composed of SiC-based reinforcements.

It may also, optionally, be of advantage to <<bring out>> thereinforcements (41) of the joint (42) between the surfaces (43, 44) tobe joined, assembled, of the parts (45, 46) to be joined, assembled inorder to facilitate initiation of the infiltration of the braze alloyinto the joint (42) as illustrated in FIG. 4.

This method is particularly recommended for composite materials such asCMC materials which are porous, in particular on their edges.

The brazing composition may be deposited for example in the form of abead of braze alloy (47) distant from the edge (48) of the part (46)i.e. at a distance of 2 to 5 mm away from the edge to allow initiationby means of the reinforcements which are brought out of, emerge (49),from the joint (42) without the risk of the braze alloy infiltratinginto the porosities of the composite material such as CMC.

The joint generally consists of at least 50% by volume of the Si—Ni—Alalloy, this alloy having a composition of between 60% and 55 atomic % ofsilicon, 30% and 34 atomic % of nickel, and 11% and 9 atomic % ofaluminium, and in general at most 50% by volume of reinforcements suchas ceramic particles or ceramic fibres (SiC or C for example).

The parts ready to be brazed are then arranged in a heating device suchas a furnace, or subjected to heating using any other suitable means.

The furnace is generally a graphite furnace, under a vacuum or in aneutral gas atmosphere, but a metal furnace may also be used.

In general the vacuum is a high vacuum i.e. the pressure is 10⁻³ to 10⁻⁵Pa, for example 10⁻⁴ Pa.

Preferably, the neutral gas is argon.

With the invention it is even possible to use argon of commercialquality (generally having 5 ppm O₂).

The parts to be joined are subjected to a heat cycle, in the furnace forexample.

For example, the assembly formed by the parts and the brazingcomposition can be brought to the brazing temperature by observing apreferably <<slow>> temperature rise, with one or more temperature rampsfrom ambient temperature.

This temperature rise can be obtained for example using a temperatureramp of 1° C. to 5° C./minute.

The brazing plateau is generally conducted at a temperature, which isthe brazing temperature, that is preferably at least 15°, morepreferably at least 30° C. higher than the melting point or liquidustemperature of the chosen brazing composition, braze alloy.

For the brazing of porous surfaces to be brazed, for example forcomposite materials whose SiC surface coating is insufficiently thick,it may be useful to conduct brazing at a temperature between theliquidus and the solidus to obtain a braze alloy in the semi-solid stateduring the brazing (temperature) plateau. The braze alloy is thenviscous and infiltration thereof into the porosities can be bettercontrolled.

This brazing temperature is generally from 1040° C. to 1150° C.,preferably 1080° C. to 1100° C., depending on the brazing compositionand the relative proportions of Ni, Al and Si in this composition.

The melting temperature of the compositions, according to anotheradvantage of the method of the invention, allows the use of the assemblyat up to 950° C. and even up to 980° C.

Surprisingly, although the brazing temperature of the brazingcompositions according to the invention is lower than 1150° C.,excellent adhesion and good wetting of the silicon carbide are obtainedwith rapid wetting kinematics, as shown by the sessile drop testsperformed with these brazing compositions, and it is therefore possible(see Example 1) to obtain a contact angle smaller than 45° after brazingfor 30 minutes at 1100° C.

This excellent wetting is indispensable to achieve good quality of theformed joints, since it ensures good quality of the filling of thejoint, but it does not always allow to ensure a good mechanicalbehaviour since this latter property is unpredictable. Yet,surprisingly, the joints prepared with the brazing compositions of theinvention also have excellent mechanical properties (cf. Example 3).

The above-defined brazing temperature (1040° C. to 1150° C., preferably1080° C. to 1100° C.), is held for a time of 1 to 150 minutes,preferably 30 to 150 minutes, more preferably 60 to 120 minutes, mostpreferably 90 to 120 minutes, this being called the brazing plateau.

For materials having relatively porous brazing surfaces such ascomposite materials, it may be useful to reduce the usual brazing timewhich is generally from 30 to 150 minutes, to a time of a few minutesnamely a time of between 1 and 30 minutes for example, to prevent toomuch infiltration of the brazing composition into the porosities of thematerial to the detriment of filling of the joint.

The duration of the brazing plateau is dependent on the size of theparts to be joined, the thickness of the joint and more specifically onthe size of the surfaces to be brazed. It is effectively possible forthis duration to reach 150 even 180 minutes for very large parts havinglarge surfaces areas to be brazed, namely typically at least 50×50 mm².

A brazing plateau for the method of the invention may for example beconducted at a brazing temperature of 1100° C. for 90 minutes.

The specific temperature of the chosen brazing plateau is a function ofthe composition of the braze alloy.

A homogenizing plateau at 980° C. for example is recommended evenessential for large-size parts (typically on and after 50×50 mm²) toguarantee the thermal homogeneity of the parts to be joined.

It is to be noted that since the wetting kinetics are good, it is notnecessary to accelerate the already excellent wetting, and this optionalfirst temperature plateau for the Ni—Al—Si compositions of the inventionis essentially even solely a homogenization plateau. This is generallyvalid for joints without reinforcement. On the other hand, this plateauis essential if reinforcements are pre-positioned between the surfacesto be brazed

This plateau can be replaced by a slow temperature rise for examplearound 1000° C.

The duration of the first plateau and the duration of the brazingplateau are dependent on the size of the furnace, the size of the partsto be brazed and the tooling supporting the parts to be brazed.

This first plateau which is therefore a homogenization plateau isgenerally observed at a temperature of 950° C. to 1000° C., for example980° C. for a minimum recommended time of one hour, for example a timeof 90 to 180 minutes, before conducting the actual brazing plateau underthe conditions already indicated above.

Both in the capillary configuration and in the <<sandwich>>configuration, the said first plateau is not indispensable for parts ofsmall size without particle reinforcements placed in the joint.

The said first plateau is generally recommended even indispensable inboth these configurations for large-size parts, namely and in generalparts which have surfaces to be brazed of more than 50×50 mm², toguarantee thermal homogeneity at the parts to be joined. It is alsocompulsory for joints with particle reinforcements.

The duration of these (temperature) plateaus can be increased, and forexample can be set at 180 minutes for the first plateau and 150 minutesfor the second plateau for parts of very large size for example having asurface area of 0.5 m² or more to be brazed.

Or else thermal homogenization may be also obtained by omitting thisfirst plateau and conducting a slow temperature rise (at the rate of0.5° C./minute for example) generally between 900° C. and 1000° C.,preferably at 980° C., so that the exposure time of the assembly in thistemperature range is for example of the order of 90 to 120 minutes.

Like the first plateau, the said slow temperature rise is advisable evenindispensable for large-size parts in both configurations.

On completion of the brazing cycle, after the brazing plateau, theassembly is cooled down to ambient temperature, at a rate of 5° C. or 6°C. per minute for example.

During cool-down, the braze alloy solidifies and the joining of theparts made of silicon carbide-based materials becomes effective whethera <<sandwich>> configuration or a <<capillary>> configuration is used.

The assemblies formed with the method of the invention were subjected tocompression/shear tests (see FIG. 6) at ambient temperature.

For sintered SiC/NiSiAl braze alloy of the invention withoutreinforcement/sintered SiC joints, the mean breaking stress valueobtained was 48 MPa which is an excellent result, much higher than thoseobtained in document [3] with a NiSi braze alloy.

For joints, substrates made of CMC composite of Cerasep A40C® type (SiCmatrix, SiC fibres)/NiSiAl braze alloy of the invention withoutreinforcement/CMC composite, the mean breaking stress value obtained wasof the order of 15 MPa, the weak point of the assembly between the brazealloy and the CMC being located at the CMC seal coat which is SiCprepared by chemical vapour deposition (CVD).

As already pointed out, this mechanical strength can be improved byadding reinforcements to the brazing composition. These reinforcementsmay be reinforcements of particle type for example in the form of a SiCpowder, or of ceramic fibre type for example in the form of fibres aloneor woven e.g. made of SiC. The reinforcement content is generally atmost 50% by volume, and may generally range from one or a few % byvolume e.g. 5% by volume up to 49% by volume of the brazing composition.As already indicated above, to obtain good filling of the joint bycapillary brazing with reinforcements pre-positioned in the joint, it isnecessary to follow a certain number of specific steps.

The assemblies of parts made of silicon carbide comprising jointsprepared using the method of the invention allow to obtain structures,apparatus, components of complex shapes having high temperatures of usewhich may reach 950° C., even 980° C., with great precision.

It is effectively known that the properties of silicon carbide:

-   -   high hardness;    -   high rigidity;    -   low density;    -   low coefficient of expansion;    -   high breaking stress;    -   good resistance to heat shock;    -   and very good conductivity makes this material an indispensable        material for present and future industrial applications, in        particular at high temperature.

In addition, SiC has very good chemical resistance to various acidsincluding hydrofluoric acid, and very good resistance to oxidation inair at high temperature of up to 1300° C.

In other words, the method of the invention can notably be applied tothe manufacture of any device, apparatus, structure, component requiringmoderately refractory joining between at least two substrates, partsmade of silicon carbide, by guaranteeing both good mechanical strengthand a satisfactory sealing, leak tightness at the joint.

This type of device, apparatus, structure, component is able to meet theneeds in various fields:

-   -   the field of heat engineering, in particular for the designing        of high performing heat exchangers since silicon carbide has        good thermal conductivity and good resistance to high        temperatures in extreme environments;    -   the field of mechanical engineering, to manufacture on-board        devices to obtain light weight, rigid, refractory components        resisting to abrasion and mechanical stresses;    -   the field of chemical engineering, since silicon carbide is        resistant to numerous corrosive chemical products such as bases        and strong acids;    -   the field of nuclear engineering, for the manufacturing of        cladding for nuclear fuel;    -   the fields of spatial optics (telescope mirror in SiC) and        aeronautics (parts made of SiC/SiC composite);    -   power electronics which use SiC substrates.

The invention will now be described using the following examplesevidently given as non-limiting illustrations.

EXAMPLES Example 1

This example describes sessile drop tests performed with a brazingcomposition, braze alloy of the invention of composition 58% Si, 32% Niand 10% Al (atomic percentages) on sintered pure α-SiC, observing asingle brazing plateau at 1100° C. for 30 minutes.

a) Preparation of the Brazing Composition and Brazing Paste

The brazing composition concerned: 58 atomic % Si, 32 atomic % Ni and 10atomic % Al was prepared from pieces of pure Si, pieces of pure Ni andpieces of pure Al.

These pieces were weighed paying heed to the proportions of the brazingcomposition and placed in an alumina crucible. The assembly was placedin a graphite furnace and subjected to a heat cycle with a plateau at1300° C. under a high vacuum for 30 minutes.

After cooling, this gave an ingot. This ingot was crushed to obtain apowder.

An organic binder (NICROBRAZ® cement) was added to this mixture ofpowders to form a viscous paste.

b) Sessile Drop Test at 1100° C.

The brazing paste thus prepared was used to form a small mound of brazealloy having a mass of approximately 50 mg. This mound of braze alloywas deposited on a previously cleaned SiC plate.

The assembly of the braze alloy mound and plate was placed in a brazingfurnace and subjected to a brazing heat cycle under a high vacuum withonly a single plateau which was the brazing plateau at 1100° C. for atime of 30 minutes.

The mound of braze alloy melts during this heat treatment and forms adrop that is called a “sessile drop”.

After cooling the wetting, contact angle of the drop was measured on thesolidified drop.

The wetting angle was of the order of 30° which corresponds to goodwetting.

The SiC and its drop of solidified braze alloy were thencross-sectioned, prepared and polished and observed under scanningelectron microscope.

The SiC/braze alloy interface did not show any reactivity on the scaleof scanning electron microscopy i.e. there was no formation of a newcompound. In particular, there was no formation of fragile compounds atthe interface.

Example 2

This example describes sessile drop tests performed with a brazingcomposition, braze alloy of the invention having the composition 58% Si,32% Ni and 10% Al (atomic percentage) on sintered pure α-SiC observing asingle brazing plateau at 1100° C. for 5 minutes.

a) Preparation of the Brazing Composition and Brazing Paste

The brazing composition concerned i.e. 58% Si, 32% Ni and 10% Al (atomicpercentages) was prepared from pieces of pure Si, pieces of pure Ni andpieces of pure Al.

These pieces were weighed paying heed to the proportions of the brazingcomposition and placed in an alumina crucible. The assembly was placedin a graphite furnace and subjected to a heat cycle with a plateau at1300° C. under a high vacuum for 30 minutes.

After cooling, this gave an ingot. This ingot was crushed to obtain apowder.

An organic binder (NICROBRAZ® cement) was added to this mixture ofpowders to form a viscous paste.

b) Sessile Drop Test at 1100° C.

The brazing paste thus prepared was used to form a small mound of brazealloy having a mass in the order of 50 mg. This mound of braze alloy wasdeposited on a SiC plate that was previously cleaned.

The assembly of the mound of braze alloy and plate was placed in abrazing furnace and subjected to a brazing heat cycle under a highvacuum with a single plateau, which was the brazing plateau at 1100° C.for a time of 5 minutes.

The mound of braze alloy melts during this heat treatment and forms aso-called sessile drop.

After cooling, the wetting, contact, angle of the drop was measured onthe solidified drop.

The wetting angle was of the order of 50° which corresponds to goodwetting.

Example 3

This example describes the preparation of bonds, joining between twoparts made of sintered pure α-SiC silicon carbide using the brazingmethod according to the invention, the brazing being conducted incapillary configuration using a brazing composition, braze alloy of theinvention composed of 58 atomic % Si, 32 atomic % Ni and 10 atomic %aluminium.

This example also describes tests, mechanical testing performed on theseassemblies.

a) Preparation of the Brazing Composition, of the Brazing Paste and ofthe Parts to be Assembled

The brazing composition concerned, namely 58 atomic % Si, 32 atomic % Niand 10 atomic % aluminium was prepared in the manner described inExample 1.

An organic binder (NICROBRAZ® cement) was added to the mixture ofpowders obtained to form a viscous brazing paste.

The parts made of sintered SiC to be assembled were plates of size 20×10mm² and thickness of 1.5 mm.

The parts were cleaned with acetone then ethanol and finally dried.

The substrates, parts were placed in contact leaving a small offset of 1to 2 mm, so as to leave a space for depositing the brazing paste in thevicinity of the joint (this configuration is called the capillaryconfiguration). The paste was deposited with a spatula on the availablesurface on the edge of the joint, in the form of a bead of braze alloy(see FIG. 2). The quantity of braze alloy was between 20 and 40 mg forthis assembly.

b) Brazing

The contacted parts ready to be brazed were placed in a brazing furnace(graphite furnace) under a high vacuum and subjected to a brazing heatcycle under a vacuum which comprised a single temperature plateau of 90minutes at 1100° C., which was the brazing plateau.

The heat cycle is illustrated in FIG. 5.

c) Observation of the Joint

After cooling, the assembly was well joined. The joint was characterizedby scanning electron microscopy. There was no <<void>> and a reactivitybetween the SiC and the braze alloy was not evidenced on the scale ofobservation under scanning electron microscopy.

d) Preparation of Mechanical Test Pieces and Results of MechanicalTesting

Assemblies, test pieces (5 test pieces) for mechanical testing wereprepared by brazing 2 parts each of size 20×10×1.5 mm³ (the thickness ofthe brazed test piece was therefore 1.5+1.5=3 mm) (61, 62) with thebrazing paste prepared at a) above and under the brazing conditionsdescribed at b) above. Since the mechanics of ceramics are statistical,more than one test piece was prepared for testing but following the samemethod of fabrication.

The test pieces are schematized in FIG. 6. They were held on a mount andsubjected to shearing during a compression/shear test (63) at ambienttemperature.

It is to be noted that this test does not allow the guaranteeing of pureshear but it is the preferred mode. However this test does allow acomparison between the assemblies.

Results of the Mechanical Tests

The breaking stresses determined for each of the 5 test pieces were 33MPa; 67 MPa; 35 MPa; 53 MPa and 54 MPa i.e. a mean of 48 MPa.

Breaking occurred in the SiC, which is characteristic of strong bondsbetween the braze alloy and the substrate in SiC.

It is to be noted that the breaking stress values of joints, assembliesof the type SiC/braze alloy with high Si content/SiC can be more or lessdispersed on account of the fragile nature of ceramic materials.

Example 4

This example describes the preparation of bonds, joining between twoparts made of CMC, more specifically made of SiC/SiC composite with aSiC matrix and SiC fibres, using the brazing method of the invention,the brazing being conducted in capillary configuration using a brazingcomposition, braze alloy of the invention composed of 58 atomic % Si, 32atomic % Ni and 10 atomic % aluminium.

This example also describes mechanical tests performed on theseassemblies.

a) Preparation of the Brazing Composition, of the Brazing Paste and ofthe Parts to be Joined

The brazing composition concerned, namely 58 atomic % Si, 32 atomic % Niand 10% aluminium was prepared in the manner described in Example 1.

An organic binder (NICROBRAZ® cement) was added to the mixture ofpowders obtained to form a viscous brazing paste.

The parts, substrates to be brazed, joined were plates of SiC/SiCcomposite with a SiC matrix and SiC fibres. The said composite materialis available from Snecma Propulsion Solide under the trade name CerasepA40C®. These plates were of size 20×10 mm² and of thickness 1.5 mm.

The parts were cleaned with acetone then ethanol and finally dried.

The substrates, parts were placed in contact leaving a small offset of 3mm, so as to leave a space for depositing the brazing paste in thevicinity of the joint (this configuration is called the capillaryconfiguration). The paste was deposited with a spatula on the freesurface at the edge of the joint, in the form of a bead of braze alloy(see FIG. 2), as described in Example 2. The quantity of deposited brazealloy was between 180 and 200 mg for this assembly.

This quantity of braze alloy is much higher than in Example 2 since theclearance between the plates made of CMC was much greater than for theplates of sintered SiC in Example 2.

For example the thickness of the joint may reach 500 μm for the CMCplates on account of the planarity defects, whereas it is generally lessthan 100 μm for the SiC plates.

b) Brazing

The parts placed in contact and ready to be brazed were placed in abrazing furnace (graphite furnace) under a high vacuum and subjected toa vacuum brazing heat cycle which, as for Example 2, comprised a singleplateau for 90 minutes at 1100° C., which was the brazing plateau.

The heat cycle is illustrated in FIG. 5.

c) Observation of the Joint

After cooling, the assembly was well joined. The joint was characterizedunder scanning electron microscopy. There was no <<void>>, and noreactivity between the SiC and the braze alloy was evidenced on thescale of observation under scanning electron microscopy.

The thickness of the joint was between 100 and 500 μm depending on theobserved zones owing to local coating defects of the CMC and planaritydefects.

d) Preparation of Mechanical Test Pieces and Results of Mechanical Tests

Assemblies, test pieces (4 test pieces) for mechanical testing wereprepared by brazing 2 parts each of size 20×10×1.5 mm³ with the brazingpaste prepared at a) above and under the brazing conditions described atb) above.

The test pieces were of similar size to those in Example 2 and weresimilarly tested under compression/shear.

Results of the Mechanical Tests:

The breaking stresses determined for each of the 4 test pieces were 14MPa; 12 MPa; 13 MPa and 20 MPa i.e. a mean of about 15 MPa.

For three test pieces, yield occurred by detachment of the SiC coating“seal coat” from the CMC. This coating therefore proves to be the weakpoint of the CMC/braze alloy/CMC assembly.

For the fourth test piece, the measured stress corresponded to the onsetof degradation of the composite.

Example 5

This example describes the preparation of bonds, joining, assemblies,between two parts made of CMC, more specifically made of SiC/SiCcomposite with SiC matrix and SiC fibres, using the brazing method ofthe invention, brazing being conducted in capillary configuration usinga brazing composition, braze alloy of the invention composed of 58atomic % Si, 32 atomic % Ni and 10 atomic % aluminium and reinforcementsof SiC particles not heat treated at 1400° C.

This example further describes tests, mechanical testing conducted onthese assemblies.

a) Preparation of the Brazing Composition, of the Brazing Paste and ofthe Parts to be Joined

The brazing composition concerned, namely 58 atomic % Si, 32 atomic % Niand 10 atomic % aluminium was prepared in the manner described inExample 1.

An organic binder (NICROBRAZ® cement) was added to the mixture ofpowders to form a viscous brazing paste.

The parts, substrates to be brazed were plates (71, 72) made of SiC/SiCcomposite with a SiC matrix and SiC fibres. Said composite material isavailable from Snecma Propulsion Solide under the trade name CerasepA40C®. These plates (71, 72) were of size 20×10 mm² with a thickness of1.5 mm.

The parts (71, 72) were cleaned with acetone followed by ethanol andthen dried.

The plates (71, 72) were coated with SiC particles of particle size 50μm. These particles were not heat treated at 1400° C.

For depositing on the composite plates, the SiC particles were bondedtogether using an organic binder such as a cement of NICROBRAZ® type,which allows the obtaining of a paste easy to deposit on the CMC plates.Depositing was carried out as indicated in FIG. 7 and the amount ofdeposited particles was 87 mg±1 mg.

The plates of CMC (71, 72) were then contacted leaving a small offset(73) of 3 mm so as to leave a space (74) to deposit the brazing paste inthe vicinity of the joint (75) (this configuration is called a capillaryconfiguration).

The joint (75) was filled with the paste of SiC reinforcement particles(76) which projected beyond the joint (75) over the available, free,surface (74) offset from the lower plate (72).

The paste of brazing composition (77) was deposited using a spatula overthe available surface (74) at the edge of the joint, in the form of abead of braze alloy (77) (see FIG. 7), as described in Example 2. Theamount of braze alloy deposited was between 195 and 220 mg for thisassembly.

This amount of braze alloy was much higher than in Example 2 since theclearance between the CMC plates was much greater than between theplates of sintered SiC in Example 2.

For example, the thickness of the joint may reach 700 μm for CMC platesowing to planarity defects whereas it is generally less than 100 μm forSiC plates.

b) Brazing

The parts placed in contact and ready to be brazed were placed in abrazing furnace (graphite furnace) under a high vacuum and subjected toa vacuum heat cycle which, as for Example 2, comprised a single plateauof 90 minutes at 1100° C., which was the brazing plateau.

The heat cycle is illustrated in FIG. 5.

c) Preparation of the Mechanical Test Pieces and Results of MechanicalTesting

Assemblies, test pieces (5 test pieces) for mechanical testing wereprepared by brazing 2 parts each of size 20×10×1.5 mm³ with the brazingpaste prepared at a) above with the coating of SiC particles describedabove, and under the brazing conditions described at b) above.

The test pieces were of similar size to those in Example 2 and weretested in the same manner under compression/shear.

Results of Mechanical Tests:

The breaking stresses determined for each of the 5 test pieces were 13MPa; 15 MPa; 14 MPa; 11 MPa and 22 MPa i.e. a mean of 15 MPa.

For four test pieces breaking occurred by detachment of the SiC coating“seal coat” from the CMC. This coating therefore proves to be the weakpoint of the CMC/braze alloy/CMC assembly.

For the fifth test piece, the measured stress corresponded to the onsetof degradation of the composite.

After these tests, the test pieces were cross-sectioned. A lack of brazealloy was observed in the centre of the test pieces.

d) Observation of the Joints

After the mechanical tests, the test pieces were cross-sectioned. A lackof braze alloy was observed (under SEM, but also visually) in the centreof the test pieces.

The thickness of the joint was between 100 and 700 μm depending on thezones observed owing to local defects of the CMC coating and planaritydefects.

Example 6

This example describes the preparation of bonds, joining, assemblies,between two parts made of CMC, more specifically made of SiC/SiCcomposite with a SiC matrix and SiC fibres, using a brazing method ofthe invention, brazing being conducted in capillary configuration usinga brazing composition, braze alloy of the invention composed of 58atomic % Si, 32 atomic % Ni and 10 atomic % aluminium, and SiC particlereinforcements that were heat treated at 1460° C.

a) Preparation of the Brazing Composition, of the Brazing Paste and ofthe Parts to be Joined

The brazing composition concerned, namely 58 atomic % Si, 32 atomic % Niand 10 atomic % aluminium was prepared in the manner described inExample 1.

An organic binder (NICROBRAZ® cement) was added to the mixture ofpowders obtained to form a viscous brazing paste.

The parts, substrates to be brazed, assembled were two plates made ofSiC/SiC composite with a SiC matrix and SiC fibres. Said compositematerial is available from Snecma Propulsion Solide under the trade nameCerasep A40C®.

The size of these plates was respectively 20×30 mm² and 20×40 mm² andthey each had a thickness of 1.5 mm.

The parts were cleaned with acetone followed by ethanol and then dried.

The plates were coated with SiC particles of particle size 50 μm.

These particles were heat treated at 1460° C. under a high vacuum fortwo hours. After this heat treatment, the SiC particles were storedunder argon until use.

For depositing on the composite plates, the SiC particles were bondedtogether with an organic binder such as a cement of NICROBRAZ® type,which allows the obtaining of a paste easy to deposit on the CMC plates.Depositing was carried out as indicated in FIG. 4, and the quantity ofdeposited particles was 194 mg±1 mg, this quantity being distributedbetween the two plates.

The CMC plates were then placed in contact leaving a slight offset of 3mm so as to leave a space for depositing the brazing paste in thevicinity of the joint (this configuration is called a capillaryconfiguration).

The joint (42) was filled with the paste of SiC reinforcing particles(41) which projected beyond the joint (42) over the available surface,offset from the lower plate (45).

The paste was deposited with a spatula over the available surface at theedge of the joint, in the form of a bead of braze alloy (47) (see FIG.4), as described in Example 2. The quantity of deposited braze alloy was1280 mg for this assembly.

This amount of paste is high since there was a large clearance betweenthe CMC plates.

b) Brazing

The parts placed in contact and ready to be brazed were placed in abrazing furnace (graphite furnace) under a high vacuum and subjected toa vacuum brazing heat cycle which comprised two temperature plateaus,namely:

-   -   a first plateau at 980° C. for 180 minutes,    -   a second plateau at 1100° C. for 90 minutes. The heat cycle is        illustrated in FIG. 3.

c) Observation of the Joints

The assembly thus prepared after step b) was cross-sectioned andcharacterized under scanning electron microscopy.

The joint was fully filled with the braze alloy, even in the centre.

This example shows that filling of the centre of the joint is controlledin the presence of reinforcement.

REFERENCES

-   [1] Gasse A., Coing-Boyat G., Bourgeois G., “Method using a thick    joint for joining parts in SiC-based materials by refractory brazing    and refractory thick joint thus obtained”, U.S. Pat. No. 5,975,407,    1999.-   [2] Gasse A., “Method for assembling parts made of materials based    on SiC by non-reactive refractory brazing, brazing composition, and    joint and assembly obtained by said method”, Patent application    US-A1-2003/0038166.-   [3] Heap H., “Method of Brazing”, U.S. Pat. No. 3,813,759, 1974.-   [4] S. Kalogeropoulou, L. Baud, N. Eustathopoulos., “Relationship    between wettability and reactivity”, Acta. Metall. Mater., Vol. 43,    N^(o) 3, pp. 907-912, 1995.-   [5] C. Rado, S. Kalogeropoulou, N. Eustathopoulos., “Wetting and    bonding of Ni—Si alloys on silicon carbide”, Acta. Metall. Mater.,    Vol. 47, N^(o) 2, pp. 461-473, 1999.-   [6] J. R. Mc Dermid, R. A. L. Drew., “Thermodynamic brazing alloy    design for joining silicon carbide”, J. Am. Ceram. Soc., Vol. 74,    N^(o) 8, pp. 1855-1860, 1991.-   [7] Montgomery F. C., Streckert H. H., Braze for Silicon Carbide    bodies, U.S. Pat. No. 5,447,683, 1995.

1. A brazing composition comprising an alloy comprising, in atomicpercentages, 45% to 65% silicon, 28% to 45% nickel and 5% to 15%aluminium.
 2. The composition according to claim 1, wherein the alloy isa ternary alloy consisting, in atomic percentages, of 45% to 65%silicon, 28% to 45% nickel and 5% to 15% aluminium.
 3. The compositionaccording to claim 1, wherein a temperature for brazing the brazingcomposition is equal to or lower than 1150° C.
 4. The compositionaccording to claim 1, wherein the said brazing composition comprises, inatomic percentages, 55% to 60% silicon, 30% to 34% nickel and 9% to 11%aluminium.
 5. The composition according to claim 1, wherein the saidbrazing composition comprises, in atomic percentages, 57.5±1% silicon,32.5±1% nickel and 10±0.5% aluminium.
 6. A composition for thenon-reactive brazing of parts comprising silicon carbide-basedmaterials, the composition comprising: the non-reactive brazingcomposition according to claim 1; and a reinforcement.
 7. Thecomposition according to claim 5, wherein the reinforcement comprisesceramics.
 8. The composition according to claim 5, wherein thereinforcement is in form of particles, of fibres, of a non-woven fabricof fibres, of a woven fabric of fibres, of a felt, or of a foam.
 9. Thecomposition according to claim 5, wherein the reinforcement is at most50% by volume relative to a total volume of the brazing composition. 10.The composition according to claim 5, wherein the silicon carbide-basedmaterials are selected from the group consisting of pure silicon andcomposite SiC-based materials.
 11. The composition according to claim 5,wherein the silicon carbide-based materials are selected from the groupconsisting of sintered pressureless silicon carbide (“PLS-SiC”), Siinfiltrated silicon carbide (“SiSiC” or “RBSC”), porous recrystallizedsilicon carbide (“RSiC”), graphite silicon (“C-SiC”) composed ofgraphite coated with a SiC layer, SiC/SiC composites, SiC/SiC compositeswith self-healing matrix, C/SiC composites, SiC monocrystals, and SiCcomposites with another ceramic.
 12. The composition according to claim5, wherein the said silicon carbide-based materials comprise a siliconcarbide content of at least 50% by mass relative to a total mass of thesilicon carbide-based materials.
 13. A refractory joint joining partscomprising silicon carbide-based materials and formed by non-reactivebrazing of a brazing composition comprising an alloy comprising, inatomic percentages, 45% to 65% silicon, 28% to 45% nickel and 5% to 15%aluminium, the joint having substantially the same composition as thebrazing composition.
 14. The refractory joint according to claim 12,wherein said alloy is a ternary alloy consisting, in atomic percentages,of 45% to 65% silicon, 28% to 45% nickel and 5% to 15% aluminium. 15.The refractory joint according to claim 12, wherein each of said brazingcomposition and said refractory joint comprises, in atomic percentages,55% to 60% silicon, 30% to 34% nickel and 9% to 11% aluminium.
 16. Therefractory joint according to claim 12, wherein each of said brazingcomposition and said refractory joint comprises, in atomic percentages,57.5±1% silicon, 32.5±1% nickel and 10±0.5% aluminium.
 17. Therefractory joint according to claim 12, wherein the refractory jointfurther comprises a reinforcement.
 18. The refractory joint according toclaim 12, wherein the reinforcement comprises ceramics.
 19. Therefractory joint according to claim 17, wherein the reinforcement is inform of particles, of fibres, of a non-woven fabric of fibres, of awoven fabric of fibres, of a felt, or of a foam.
 20. The refractoryjoint according to claim 17, wherein the reinforcement is at most 50% byvolume relative to a total volume of the brazing composition.
 21. Therefractory joint according to claim 12, wherein said refractory joint iscapable of withstanding a temperature up to 980° C.
 22. The refractoryjoint according to claim 12, wherein the refractory joint is formed bycapillary brazing of the brazing composition and therefore substantiallyfills pores present on the parts.
 23. An assembly comprising: at leasttwo parts comprising silicon carbide-based materials; and the refractoryjoint according to claim 12 joining said at least two parts.
 24. Theassembly according to claim 22, wherein the silicon carbide-basedmaterials are selected from the group consisting of pure silicon andcomposite SiC-based materials.
 25. The assembly according to claim 22,wherein the silicon carbide-based materials are selected from the groupconsisting of sintered pressureless silicon carbide (“PLS-SiC”), Siinfiltrated silicon carbide (“SiSiC” or “RBSC”), porous recrystallizedsilicon carbide (“RSiC”), graphite silicon (“C-SiC”) composed ofgraphite coated with a SiC layer, SiC/SiC composites, SiC/SiC compositeswith self-healing matrix, C/SiC composites, SiC monocrystals, and SiCcomposites with another ceramic.
 26. The assembly according to claim 22,wherein the silicon carbide-based materials comprise a silicon carbidecontent of at least 50% by mass relative to a total mass of the siliconcarbide-based materials.
 27. The assembly according to claim 22, whereinsaid alloy is a ternary alloy consisting, in atomic percentages, of 45%to 65% silicon, 28% to 45% nickel and 5% to 15% aluminium.
 28. Theassembly according to claim 22, wherein each of said brazing compositionand said refractory joint comprises, in atomic percentages, 55% to 60%silicon, 30% to 34% nickel and 9% to 11% aluminium.
 29. The assemblyaccording to claim 22, wherein each of said brazing composition and saidrefractory joint comprises, in atomic percentages, 57.5±1% silicon,32.5±1% nickel and 10±0.5% aluminium.
 30. The assembly according toclaim 22, wherein the assembly further comprises a reinforcement. 31.The refractory joint according to claim 29, wherein the reinforcementcomprises ceramics.
 32. The assembly joint according to claim 29,wherein the reinforcement is in form of particles, of fibres, of anon-woven fabric of fibres, of a woven fabric of fibres, of a felt, orof a foam.
 33. The assembly according to claim 29, wherein thereinforcement is at most 50% by volume relative to a total volume of thebrazing composition.